Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase.

Chloroplast ATP synthase (CFCF) synthesizes ATP by using a proton electrochemical gradient across the thylakoid membrane, termed ΔμH, as an energy source. This gradient is necessary not only for ATP synthesis but also for reductive activation of CFCF by thioredoxin, using reducing equivalents produced by the photosynthetic electron transport chain. ΔμH comprises two thermodynamic components: pH differences across the membrane (ΔpH) and the transmembrane electrical potential (ΔΨ).

Dissipation of the proton electrochemical gradient in chloroplasts promotes the oxidation of ATP synthase by thioredoxin-like proteins.

Chloroplast FF-ATP synthase (CFCF) uses an electrochemical gradient of protons across the thylakoid membrane (ΔμH) as an energy source in the ATP synthesis reaction. CFCF activity is regulated by the redox state of a Cys pair on its central axis, that is, the γ subunit (CF-γ). When the ΔμH is formed by the photosynthetic electron transfer chain under light conditions, CF-γ is reduced by thioredoxin (Trx), and the entire CFCF enzyme is activated.

The Importance of the C-Terminal Cys Pair of Phosphoribulokinase in Phototrophs in Thioredoxin-Dependent Regulation.

Phosphoribulokinase (PRK), one of the enzymes in the Calvin-Benson cycle, is a well-known target of thioredoxin (Trx), which regulates various enzyme activities by the reduction of disulfide bonds in a light-dependent manner. PRK has two Cys pairs conserved in the N-terminal and C-terminal regions, and the N-terminal one near the active site is thought to be responsible for the regulation. The flexible clamp loop located between the N-terminal two Cys residues has been deemed significant to Trx-mediated regulation.

Chloroplast ATP synthase is reduced by both f-type and m-type thioredoxins.

The activity of the molecular motor enzyme, chloroplast ATP synthase, is regulated in a redox-dependent manner. The γ subunit, CF-γ, is the central shaft of this enzyme complex and possesses the redox-active cysteine pair, which is reduced by thioredoxin (Trx). In light conditions, Trx transfers the reducing equivalent obtained from the photosynthetic electron transfer system to the CF-γ.